Cardiac defibrillators and pacemakers are commonly designed to be implanted within a human patient. The cardiac defibrillators include an electrical energy storage component as part of a power supply designed to provide repeated burst discharges of several joules of electrical energy. Cardiac pacemakers supply lower energy bursts but much more frequently. Both devices therefore require energy storage components of large capacity in order to reduce the number of occasions when the device must be explanted to renew its energy storage component. It is advantageous that the energy storage component be compact and capable of large energy storage. It is advantageous if the energy storage component can be configured to the shape of the overall device which is typically a flat disc shaped configuration to facilitate implantation subcutaneously in the patient. It is well known that electrolytic capacitors have some properties that are suited for this purpose.
Electrolytic capacitors are normally made by tightly winding an etched aluminum foil anode, a Kraft paper or fabric gauze separator and cathode and holding the roll together with an adhesive tape. Connections to the anode and electrode are made via tabs.
Such capacitors are inherently thick and where they employ liquid electrolyte are subject to leakage, which can damage electrical components and lead to failure of the device. Sealing the device hermetically is not an adequate solution of this problem because of gases may build up within the device. Expansion chambers to receive the gases have been provided to deal with such problems, but that has lead to the disadvantage of even a larger size for the capacitor. Electrolytic capacitors rely upon an oxide layer that forms on the typically aluminum anode. This oxide layer is a dielectric layer between the anode and cathode. The liquid electrolyte causes the de-forming of the aluminum oxide layer present on the aluminum anode. Although the potential across the electrodes can result in currents that re-form the oxide layer, the de-formation results in a shorter lifetime of the formed oxide layer.
Polymeric material between the anode and cathode is known. U.S. Pat. No. 3,555,369 proposed replacing the kraft paper spacer with a semipermeable membrane of a polymeric material. However this required that the membrane be impregnated with a solvent-based liquid electrolyte, required the hermetic sealing and the provision of expansion chambers to deal with the gas generated, and left unsolved the problem of the de-forming of the oxide layer of the capacitor.
Polymeric materials that behave similar to a solid electrolyte are also known. U.S. Pat. No. 3,883,784 proposed to produce capacitors employing a polymeric material having ionic acceptors and ionic donors. This patent suggested interposing the polymeric material in a film in place of the kraft spacer. Since the film was in fact thicker than the kraft space it did not contribute to a reduction in the size of the capacitor. Further, the '784 patent did not disclose an electrolytic capacitor and was unlikely to be capable of supporting electrolytic action at over voltages or have any ability to re-form the oxide layer on its anode. The capacitor of the '784 invention had a capacitance much smaller than electrolytic capacitors of comparable size.
Solid electrolytes are also known. U.S. Pat. No. 4,942,501, and its continuations, U.S. Pat. Nos. 5,146,391 and 5,153,820 provided an electrolytic capacitor that employed, between its anode and cathode, a layer of solid electrolyte comprising a solid solution of certain metal salts in a polymer matrix. These capacitors were immune to leakage and were smaller than prior electrolytic capacitors of comparable construction and operating properties. The preferred method of constructing the capacitors was to deposit the polymer electrolyte onto the surfaces of the anode and polymerizing the material. The cathode was formed by deposition upon the surface of the solid electrolyte layer. The construct was then wound onto a cardboard core tube into a cylindrical body and then inserted into a cylindrical housing which was sealed by an end plate.
Flat capacitors are known. U.S. Pat. No. 4,267,566 to Moresi, Jr. disclosed a capacitor comprising a planar layered structure of anode plates, cathode plates and a paper separator. This was placed in a polymeric envelope to contain the electrolyte and the structure was encased in a hermetically sealed housing. The electrolyte that was preferred was based on the liquid solvent ethylene glycol. A similar structure is describe in Research Disclosure 33236 (Published December 1991). One problem with this type of capacitor is that the cathode and anode plates must be kept in intimate contact and `connected` by the electrolyte. This has, prior to the present invention, required a clamp as described in Research Disclosure 33236 (Published December 1991), which adds to the complexity in manufacture of the capacitor and also increases the volume of the capacitor.
Japanese Patent Application No. JP 4-184811 to Mitsubushi also discloses a solid electrolyte film having both ionic and electronic conductivity which is suitable as an electrolyte for electrolytic capacitors. The patent discloses using a porous film or a fabric integrated with the polymer to increase the mechanical and physical strength of the solid electrolyte. The details of the construction still require the use of binding materials to complete the construction.